1. Field of the Invention
This invention relates to photostorage and emissive materials, and particularly colored photostorage materials which have a long emission decay time and achieve a balance between daytime brightness and night time glow in the dark characteristics.
2. Prior Art
Photostorage materials absorb energy from sunlight, fluorescent light and other light sources that excite the material and convert that energy into visible light rays and other radiation. Photostorage materials continue to re-emit light and other radiation even after excitation ceases and is capable of repeating this process over a prolonged period of time.
One common photostorage material which exists in the prior art comprises copper activated zinc sulfide, such as ZnS:Cu. Such a material is frequently used in such products as watches, clocks and safety signs. In these types of photostorage materials, the zinc sulfide absorbs energy from ultraviolet rays and is activated sequentially.
However, copper activated zinc sulfide has a disadvantage in that its brightness decreases very rapidly and it deteriorates when exposed to ultraviolet radiation in the atmosphere. In particular, when copper activated zinc sulfide is used in watch dials, it only provides visible brightness for 20 to 30 minutes and cannot be used or exposed for long periods of time to the sun. In addition, the copper activated zinc sulfide generally has the appearance of a light yellow-green color in day light and is not very aesthetically pleasing.
Another method of providing glow in the dark properties is to use a radioactive substance such as promethium. Promethium will emit light for long periods of time, but has other disadvantages. In particular, promethium as a radioactive material is harmful to the human body and requires costly handling procedures. Associated costs include licensing to possess and distribute this hazardous material as well as waste disposal problems.
Recently, photostorage materials activated by normal light energy and having a long decay time have been developed. An example of such a material is shown in Chinese Patent Application Publication Number CN10537807A. The long decay photostorage material of this Chinese patent can be represented by the formula: EQU m(Sr.sub.1-x Eu.sub.x)O.nAl.sub.2 O.sub.3.yB.sub.2 O.sub.3 EQU (1&lt;m&lt;5, 0.001&lt;8&lt;0.1 1&lt;n&lt;8, 0.001&lt;y&lt;0.35)
The above Chinese photostorage material utilizes aluminum oxide, strontium oxide, boron oxide and europium oxide.
Other photostorage materials with long decay times are documented in U.S. Pat. No. 5,376,303, Japanese Patent Application Publication Number 96-170076 and Japanese Patent Application Publication Number 96-127772. While the preceding photostorage materials provide a long decay time such as 10 hours, they each have the disadvantage that they are low in initial brightness.
To overcome the disadvantages of these prior art long decay time photostorage materials, a long decay time material having initial high brightness was developed by Chemitec in under the trademark Picariko.TM. in Japan and PERMAGLOW.RTM. the United States. This photostorage material is in two types identified as CP-05 which glows green, and CP-10 which glows blue.
PERMAGLOW.RTM. is a synthetic super luminescent crystal composed of aluminum oxide, strontium oxide, calcium oxide, europium oxide and boron oxide. The methods of manufacturing synthetic crystals with photostorage properties and practical applications for PERMAGLOW.RTM. are described in U.S. patent application Ser. No. 08/703,152, now U.S. Pat. No. 5,885,483.
This unique luminescent material accumulates light for high initial brightness and long afterglow duration. It absorbs light from the sun, fluorescent lights, and other light sources that excite it and glows for up to 10 hours.
PERMAGLOW.RTM. was previously available in only two colors that glow in the dark; green glow and blue glow. Both have a light yellow green in day light conditions. When compared to the prior art, PERMAGLOW.RTM. provides superior performance including:
higher initial brightness PA1 up to 10 hours luminescence from each exposure to light PA1 excellent resistance to environmental conditions, including sunlight PA1 superior chemical resistance PA1 long product life of 10 years
While PERMAGLOW.RTM. overcomes a significant number of problems of the prior photostorage materials, this luminescent material has its own disadvantages. In particular, both PERMAGLOW CP-05 and CP-10 appear light yellow-green in daylight conditions and are not very aesthetically pleasing in daylight. Also, when exhibiting glow in the dark characteristics, only the two colors, namely green and blue, are available from the PERMAGLOW CP-05 and CP-10.